An electric storage device providing high voltage and a high energy density is required as a power source for driving electronic equipment. For such a use, for example, lithium-ion secondary cells and lithium-ion capacitors are expected.
The electrode used in an electric storage device is usually produced by applying a composition (slurry for an electrode) comprising an active material and a polymer functioning as an electrode binder onto a current collector surface and drying it. The polymer used as the binder for an electrode is required to have, for example, the following characteristics:
(1) capability of binding active materials to each other and capability of adhering an active material to a current collector;
(2) resistance to friction in winding of the electrode; and
(3) “tolerance to powder falling” so that, fine powder of an active material does not fall from the coating film of an applied and dried composition (hereinafter, also simply referred to as “active material layer”) even in the subsequent steps such as cutting.A polymer satisfying these various demand characteristics can increase the degree of freedom in structural design (e.g., an electrode folding process and an electrode winding radius) of an electric storage device, resulting in achievement of a reduction in size of the device.
It has been known from experience that the degrees of the capability of binding active materials to each other, the capability of adhering an active material to a current collector, and the tolerance to powder falling are approximately proportional to each other. Accordingly, throughout the specification, these characteristics may be comprehensively referred to as “adhesion”.
Recently, in order to achieve such demands for a higher output and a higher energy density of an electric storage device, application of a material having a high lithium occlusion capacity as an electrode active material has been investigated. For example, silicon in a form of an intermetallic compound with lithium can reversibly occlude and discharge lithium. The theoretical maximum capacity of this silicon is about 4,200 mAh/g, which is considerably high compared to the theoretical capacity, about 370 mAh/g, of carbon materials conventionally used. Accordingly, the capacity of an electric storage device will be drastically increased by using a silicon material as the negative-electrode active material. However, since silicon materials show large volume changes associated with charge and discharge, when the conventionally used electrode binder material is directly applied to a negative-electrode active material employing a silicon material, the initial adhesion is not maintained to cause a defect of significantly reducing the capacity in association with charge and discharge.
Methods of using polyimide as an electrode binder for holding such a silicon material in an active material layer have been proposed (Japanese Patent Laid-Open Nos. 2007-95670, 2011-192563, and 2011-204592). These technologies are based on a technological thought for preventing the change in volume of a silicon material by restraining the silicon material with the rigid molecular structure of a polyimide. These patent documents describe that the polyimide is generated by applying an electrode-forming slurry comprising a polyamic acid onto a current collector surface to form a coating film and heating the coating film at a high temperature to thermally imidize the polyamic acid. However, the binder utilizing such a polyimide has insufficient adhesion, and therefore the electrode is degraded by repeating charge and discharge and does not exhibit sufficient durability.